The invention relates to a nonwoven fabric having a smooth surface, especially for use as underlayer material for semipermeable membranes.
It is known that semipermeable membranes, which find application in apparatus operating on the principle of reverse osmosis and ultrafiltration, and are used in the form of tubular and flat membranes, are structurally weak and consequently require a structural support. This support is provided, for example, by perforated tubes or perforated or channeled plates. Since the spacing of the holes or channels in such support materials must be relatively great for the sake of strength on account of the high pressures required in the process, a porous mass which is not compressible in service is used as a drainage stratum between the semipermeable membrane and the support means. Usually, this drainage stratum, which will be referred to hereinafter as the underlayer, is directly coated with the membrane, and serves both for the drainage of the permeate and for the technological support of the delicate membrane.
It is known to use as underlayers nonwoven fabrics which are produced by dry or wet methods or by the spun mat method, and which have been consolidated in some cases by hot calendering. In all nonwovens of this kind, the protrusion of individual fiber ends or loops, however, cannot be prevented entirely. On account of the in the micron-range yet relatively coarse fibers which can be worked by these methods, the surface of the fabric is still relatively rough, even after calendering. On account of this roughness of the underlayer, a membrane of irregular thickness is produced when the underlayer is coated, and this results in irregular membrane properties in the micron range. Due in particular to the protruding fibers, difficulties occur when the membrane is applied in that, during the coating process, the fibers are not laid down and embedded by the membrane solution, but protrude to a greater or lesser extent through the fabric surface and thus extend into the membrane and, in some cases, all the way into and through the surface of the active membrane which is very thin, having a thickness between 500 and 5000 Angstroms. The result of this is a membrane having irregular characteristics and insufficient permeability. Since in the applications in which semipermeable membranes of this kind are used, material flaws must in general be excluded, efforts have been made to increase the thickness of the layer of cellulose acetate, for example, applied by extrusion, far beyond the optimum. This involved not only a reduction in the rate of permeation of the membrane, but also a considerable increase of production costs.